The towering "Detroit Industry" murals by Mexican artist Diego Rivera donated to the city weren’t among the Detroit Institute of Arts works appraised by New York-based Christie’s for possible liquidation. The company assessed only pieces purchased with city money; those are worth between $452 million and $866 million.

As Detroit’s bankruptcy heads into negotiations to buzz-cut $18 billion in debt, the fate of the collection, with works by Bruegel, van Gogh and Cezanne, has drawn worldwide interest. The Rivera frescoes cover 4,032 square feet as the museum’s signature work. They were commissioned in 1932 by Edsel B. Ford, then president of Ford Motor Co., for $20,800, according to the institute. They’re also part of the building.

Orr ordered the appraisal as part of a tally of municipal assets. He’s said he doesn’t want to sell art, though he wants to leverage its value to help pay for a settlement.

Christie’s suggested five alternatives to selling the city’s art to pay debts, such as using it as collateral for loans or selling some to philanthropists with the condition that it remain on display in Detroit.

The art, building and grounds are owned by the city and operated by a nonprofit corporation. Christie’s assessed 2,781 of the museum’s about 66,000 items, those that the city purchased, rather than receiving as donations. Eleven works account for 75 percent of the art’s appraised value according to the company’s preliminary report to Orr. The Rivera frescoes aren’t among them, Elizabeth Van Bergen, a Christie’s spokeswoman, said in an email.

The 27 panels, created in eight months, are the museum’s calling card. The largest is 75 feet long and 17 feet high.

They often draw gasps from viewers who enter the indoor court that holds them. Kinetic depictions of multiracial workers in a car factory are surrounded with symbolic, surreal images such as an unborn baby in a plant bulb, female sentries representing four races of mankind and the effects of chemical warfare.

A factory stamping press is depicted as the Aztec goddess of creation, Coatlicue.

The work was unveiled in 1933 and stirred protests from some who objected to Rivera’s communist views, his depiction of different races and female nudes in the frescoes, according to a museum summary. Rivera’s artist wife and fellow communist, Frida Kahlo, took photographs of a Ford plant that were used to design them.

After the Detroit project, Rivera was commissioned by Nelson A. Rockefeller in 1933 for a 63-foot-long mural at Rockefeller Center in New York that ignited outcry over its leftist themes and was destroyed.

After their encounters with titans of capitalism, Kahlo and Rivera gave asylum in their Mexican home to exiled Bolshevik Leon Trotsky and his wife, Natalia, for two years in the late 1930s.

In the decades since, Detroit, a former worldwide capital of manufacturing and a stronghold of labor agitation has fallen on hard times as its factories closed and it lost almost half its population, dwindling to about 700,000 inhabitants.

The Christie’s appraisal shows that its art would fetch a relatively small amount, said Michael Bennett, who teaches intellectual property law at Northeastern University in Boston and is a board member at an art gallery there. Bennett attended the Dec. 3 hearing in Detroit where U.S. Bankruptcy Judge Steven Rhodes ruled the city eligible for bankruptcy.

"If the city were to sell off the masters, liquidate its collection, it would send a signal to the world it was cashing in on its cultural cachet," Bennett said in a phone interview. "Its presence on the world stage would be diminished. It would cease to be among the upper echelon of world cities with respect to art."

"Detroit needs the art now more than ever to comfort its citizens."

The museum, in opposing a forced sale, cites an opinion by Michigan Attorney General Bill Schuette that the art is a public trust. In a statement Dec. 4, the museum called on Orr to protect it.

"The museum collection is a cultural resource, not a municipal asset," said the museum in a statement explaining why it wouldn’t comment on the Christie’s report.

Nowling said creditors may challenge Orr’s stand that only art bought with taxpayer money — mostly during the first half of the 20th century — is exposed to a sale. He said donated art often came with covenants prohibiting sale or transfer.

"Once you get below the level of straight-out, city- purchased art, it gets kind of murky," Nowling said. "Creditors are going to dispute everything."